The four-crate shipment was on its way from China to Egypt, the Swiss Agency for Therapeutic Products said.

The tablets are counterfeit versions of Xanax - a drug manufactured by Pfizer to treat severe anxiety or panic disorder.

Tests showed the drugs contained no active ingredients, but would be very difficult to recognise as fakes.

The counterfeit drugs were packed into four crates which weighed over 400kg, said the agency, also known as Swissmedic.

"Analyses in the Swissmedic laboratory revealed that the drugs, which are prescribed to treat symptoms of acute anxiety, contained no active ingredients whatsoever," Swissmedic said in a statement, adding that they were being destroyed.

"According to experts, the drugs would be unrecognisable as counterfeits at first glance."

Developing countries such as Egypt are considered a major market for the multi-billion dollar trade in fake drugs.

The World Health Organization says many countries in Africa, and parts of Asia and Latin America, have areas where more than 30% of medicines on sale can be counterfeit.

Counterfeit medication ranges from painkillers to treatment for life-threatening conditions, it says,